December 14, 2016 – Wet and windy weather with high-elevation snow continues into Friday. Freezing temperatures possible this weekend. Flood Watch through late Thursday night, Wind Advisory from noon today to noon Thursday and Winter Storm Watch from Thursday afternoon through Friday afternoon in effect.
Discussion
Amplifying ridge has shifted the large area of precipitation further to the north overnight with half inch to one inch rain amounts common across the northern mountains and north end of the Sacramento Valley. Lighter amounts fell to about as far south as I-80.
Partial clearing has allowed for some fog development in the valley to the south of Sacramento. Current temperatures remain quite mild generally ranging from the upper 30s to lower 40s in the mountains to the mid 40s to lower 50s elsewhere.
The next phase of current wet pattern will develop later today and tonight as the frontal system off the northwest coast a California begins to slowly shift back to the southeast spreading heavier precipitation and stronger winds inland as it does.
Rain rates will be on the upswing Thursday and Thursday night and will possibly be comparable to what we saw this past weekend. As such, urban, small stream, and burn scar flooding potential will be much higher. Winds will also be a factor as gusts of 40-60 mph will be possible, starting in the northern Sacramento Valley this afternoon and shifting southward later tonight into Thursday. Wind gusts this strong may cause downed trees and local power outages.
Snow levels will initially start above 7000 feet today, but fall rapidly as the front passes through. By early Friday, snow levels could drop down to 2000-4000 feet, but heavier precipitation will be winding down. There is still some uncertainty with how snow accumulation the mid-slopes of the northern Sierra will get, but the pass levels will see at least a foot and likely more.
A colder and drier pattern settles into the region by Saturday as the storm shifts to the east and northwesterly flow envelops the region. Freezing temperatures will once again be possible in the Central Valley by Saturday morning.
Extended discussion (Sunday through Wednesday)
High pressure over the eastern Pacific and west coast will keep most of the extended period dry. Pressure gradient between low pressure over the southwest U.S and the ridge off the coast will bring breezy north winds the early part of next week.
Cool northerly flow and clearing airmass will make of a cold night Saturday night into Sunday morning with current forecast putting valley minimums down into the mid 20s to around 30 degrees.
Daytime highs will be warming reaching near normal by Monday. Upper ridging begins to break down by next Tuesday as a Pacific weather system moves towards the Pacific Northwest. Models begin to diverge at this point with significant timing differences.
ECMWF drags the Pacific system across far Norcal on Tuesday afternoon. GEM is a bit slower while the GFS keeps this system completely offshore through Tuesday afternoon. Have leaned towards the ECMWF this morning adding some light showers to the northern most zones Tuesday afternoon.
ECWMF and GEM leave Norcal high and dry under upper ridging Wednesday while GFS finally gets around to bringing the Pacific system inland. With such big differences in the current modeling, confidence in this time period is low and have kept some shower threat over the northern mountains for the middle of next week.